Understanding how people make decisions about preventive health behavior is crucial to the design of effective interventions to promote such behavior. This proposal explores decision making about influenza vaccination, an important preventive health behavior because it both saves money and prevents the considerable morbidity and mortality caused by the flu. Unlike most previous studies of flu shot acceptance, the current study will use a longitudinal design over three flu seasons, allow us to address research questions previously not considered. It will also assess vaccination decisions among two previously understudied groups: healthcare workers and healthy adults in a workplace setting. These two groups are important because the flu shot has considerable health-related and economic benefits for them, and they afford a sensitive examination of decision making factors that drive vaccine acceptance, since they have easy access to free vaccination against influenza and are aware of its availability. Using a utility theory framework, the proposed study will address three Specific Aims. (1) First, we extend previous research showing that prior vaccine acceptance is one of the best predictors of present vaccine acceptance by using a longitudinal design to test two possible explanations for this association. One is that people make an explicit decision each year as to whether to obtain the vaccine, but, because costs and benefits of the vaccine remain constant from year to year, decisions are consistent across time. An alternative explanation is that vaccination is driven by habit, not explicit decisions. (2) Our second aim concerns reasons for the strong predictive relationship, found in previous studies, between perceived effectiveness of the influenza vaccine and vaccine acceptance. We will extend previous research by examining the origins of such effectiveness beliefs. (3) Third, because the benefits of vaccination are both belayed and uncertain, we will examine the psychological role of time and risk in decisions about vaccine acceptance. We will test whether vaccine acceptance is related to individual time preferences (the extent to which future health outcomes are valued relative to certain ones). In addition, vaccination decisions may be influenced by perceptions of the risk of influenza, and intentions to vaccinate sometimes may not be activated on because of inaccurate forecasting of future preferences. The results of this study will extend the understanding of predictors of influenza vaccine acceptance and will prove valuable in the development of intervention strategies to boost vaccine acceptance rates.